The traditional use of digital display devices for reproduction or representation of continuous-tone imagery has been hampered somewhat by inefficiencies in spatial resolution and inaccurate reproduction caused by the limited number of available greyscale steps or levels in most display devices. A display device with a limited number of greyscale steps does not accurately represent an image to the observer. Gradual variations in intensity levels may appear to have a banded structure and low modulation features may be lost entirely. The performance of the display may be improved by rescaling the image to use the available levels to the best advantage. This, however, is often not sufficient to solve the problem. Alternative grouping of pixels has been used to enhance the accuracy of reproduction of images; however, a loss of resolution still occurs. It is desirous, therefore, to improve the quality and distinction of grey scale resolution in such devices.
Recent attempts to increase greyscale resolution have included varying the duration of an input signal generator in order to vary the duration of the actual input signal (U.S. Pat. No. 3,526,711, Sept. 1, 1970, T. J. DeBoer, "Device Comprising a Display Panel Having a Plurality of Crossed Conductors Driven by an Amplitude to Pulse Width Converter"), and varying the length of time that a bi-stable element remains activated (U.S. Pat. No. 3,590,156, June 29, 1971, Richard A. Easton, "Flat Panel Display System with Time-Modulated Grey Scale"), thereby increasing the quantity of greyscale levels available for image reproduction. These approaches require extensive peripheral memory capabilities.
Additional improvement techniques have included the use of three source element matrices, each of which may be activated independent of, or simultaneously with, each of the other source matrices. These source matrices are stacked on top of one another and are separated by attenuating layers. The factorial result for three such source matrices, is seven potential greyscale steps ( U.S. Pat. No. 3,626,241, Dec. 7, 1971, Dinh-Tuan Nho, "Grey Scale Gaseous Display"). This approach has proven to be complex and expensive.
The most effective gains in accuracy of reproduction have resulted from the use of a plurality of display cells or source elements to represent each image sample (U.S. Pat. No. 3,845,243, Oct. 29, 1976, Schmersal et al., "System for Producing a Grey Scale With a Gaseous Display and Storage Panel Using Multiple Discharge Elements"), rather than the traditional one to one sampling ratio. This technique is referred to as P.S.A.M. (Pulse Surface Area Modulation). In such applications, a single image sample is represented by an entire superpixel component, composed of several individual pixel elements. While this approach substantially increases the available quantity of non-zero grey levels, such applications suffer inherent drawbacks. For example, a substantial loss of resolution occurs since a larger screen area is necessary to reproduce a single image sample. Consequently, the quantity, complexity and cost of necessary hardware is substantially increased.
Each individual pixel element has an associated set of threshold levels. The actual number of threshold levels for each individual pixel element may vary according to the specific matrix configuration. As the number of threshold levels per pixel element is increased, both the efficiency and cost of the display device increase considerably. While the overall number of threshold levels for each particular element is variable, the quantity and values of each set of threshold levels are identical for all of the pixel elements. In addition, the incremental variance between threshold levels remains uniform throughout the set. Consequently, all of the pixel elements of a superpixel component respond identically to a single image sample. This retention of uniform incremental increases in threshold variances of each individual pixel element and the cost associated with providing additional grey level thresholds substantially limits the number of available grey level steps. This causes a limitation in the accuracy of reproduction of continuous tone imagery. The reproduced image, therefore, while representative of the original image, is not an entirely accurate reproduction.
In addition, the relationship of the uniform threshold coding to the positioning of the pixel elements associated with those threshold codes creates and illuminance pattern that is potentially detrimental to accurate digital reproduction of continuous tone imagery. Another disadvantage of such applications includes the necessity for expensive, complex peripheral memory/storage components.
Increasing the number of available grey level steps in such devices by increasing the number of threshold levels for each individual pixel element would be complex and costly. In addition, certain applications, by their inherent nature, prevent the inclusion of additional threshold levels. Such applications include gas discharge devices, bi-stable elements and drilling of metalized mylar film, in which the quantity of available threshold levels is limited by the physical characteristics of the device.
Increasing the ratio of the pixel elements used to represent each image sample would likewise be complex and costly and would result in a loss of resolution as a result of the larger screen area necessary to reproduce each image sample.
Further attempts to enhance the techniques discussed above have included experimentation with the configuration of the cells or source elements, in order to vary the geometric shape of each image sample representation. Various geometric image configurations have included half-tone dots, half-tone uniform rings, half-tone annular rings, and multiple half-tone concentric rings. Each of these types of geometric configurations possesses certain spatial resolution characteristics and properties. The half-tone ring representation is currently considered the most advantageous for the accurate reproduction of continous tone imagery.
While each of these approaches has improved the digital display process considerably, a still higher level of accuracy in the reproduction of continous-tone imagery by the use of a practical cost-efficient device is still needed.